1. Field of the Invention
The invention is directed to a reduced noise optical telecommunication system, and particularly to a reduced noise system incorporating an optical waveguide fiber having total dispersion that varies along the fiber length.
2. Technical Background
To achieve very long length, high data rate optical communication systems, methods for dealing with an accumulation of noise have been proposed and tested. The methods generally focus on regeneration and reshaping of optical signal pulses without resorting to costly, high maintenance, optical to electronic converters. Such electronic converters essentially filter the accumulated noise from the signal, amplify the signal, and reintroduced into the optical system the signal essentially in its originally launched condition. An all-optical solution to the problem of accumulated noise is attractive because optical components are typically less complex and more reliable than those of the optical to electronic converters (sometimes called regenerators in the art).
Sources of noise are integral to most optical systems in that noise arises from spontaneous emission in optical amplifiers, from multiple reflections at splice and connector interfaces, or even from interaction of the signal with the waveguide medium. Thus, strategies for reducing noise usually involve operations directly upon the pulse together with band pass filtering which serves to remove noise without appreciably effecting signal power. Examples of such strategies used in conjunction with soliton signaling include use of amplitude depend gain amplifiers and synchronous shaping of the solitons using, for example, a high speed optical modulator. These strategies are not ideal because the effect of the filtering on signal power is usually significant enough to require the addition of an optical amplifier. In addition, synchronous shaping techniques require complicated electronic clock recovery systems which are expensive. The introduction of additional devices into an optical system adds sources of attenuation and dispersion which must be compensated by other additional components such as optical amplifiers and dispersion compensation devices. In effect, the components used to selectively preserve the signal pulse while filtering noise typically add cost and complexity.
A more efficient solution to the problem of reducing noise in long length, high data rate systems would incorporate optical waveguide fiber. Optical waveguide fiber can be made low in attenuation, relatively low in dispersion, and is compatible with other devices in the optical system in terms of splicing and connecting. In addition, an optical waveguide fiber, as a passive component would not add to maintenance cost of the optical system. The small size and flexibility of an optical waveguide fiber allows it to be configured in any of several packages compatible with the space typically available at the end of an optical communication system.